Refactor utils.fetch to a module and mock it with nock

Add redirect support in utils.fetch

__tests__/fetch.test.ts

@@ -0,0 +1,39 @@
+import * as fetch from '../src/fetch';
+import nock = require('nock');
+
+it('checking fetch', async () => {
+  const host_url = 'https://example.com';
+  const manifest_url = host_url + '/manifest';
+  const ping_url = host_url + '/ping';
+
+  nock(host_url)
+    .get('/manifest')
+    .reply(200, {latest: 'latest'})
+    .get('/manifest', '', {
+      reqheaders: {authorization: 'Bearer invalid_token'}
+    })
+    .reply(401, {error: '401: Unauthorized'})
+    .get('/ping')
+    .twice()
+    .reply(301, undefined, {
+      Location: host_url + '/pong'
+    })
+    .get('/pong')
+    .reply(200, 'pong');
+
+  let response: Record<string, string> = await fetch.fetch(manifest_url);
+  expect(response.error).toBe(undefined);
+  expect(response.data).toContain('latest');
+
+  response = await fetch.fetch(ping_url, '', 1);
+  expect(response.error).toBe(undefined);
+  expect(response.data).toContain('pong');
+
+  response = await fetch.fetch(ping_url, '', 0);
+  expect(response.error).toBe('301: Redirect error');
+  expect(response.data).toBe(undefined);
+
+  response = await fetch.fetch(manifest_url, 'invalid_token');
+  expect(response.error).not.toBe(undefined);
+  expect(response.data).toBe(undefined);
+});